1. Field of the Invention
This invention relates to a process for the safe storage of sodium percarbonate in a bulk container that is substantially closed to the ambient atmosphere.
2. Description of the Background
The storage of sodium percarbonate (Na2CO3xc2x7.1.5H2O2) in bulk containers, such as stationary and transportable bulk containers, silo works and bunkers for bulk material, requires particular attention to the issue of safety, because sodium percarbonate can decompose spontaneously. The capacity of sodium percarbonate to spontaneously decompose during storage in a silo can lead to heating of the product and uncontrolled thermal decomposition. Sodium percarbonate decomposes in an exothermic reaction to form soda, water and oxygen. The present inventors have found that silo storage conditions, particularly elevated storage temperatures, can lead to a rapid rise in temperature and maximal temperatures of 150 to 180xc2x0 C. at the end of the decomposition reaction.
It is known that sodium percarbonate can be coated with one or more stabilizing covering layers in order to stabilize the product in the presence of detergent components, such as zeolites. However, this covering has only a limited influence on the capacity of sodium percarbonate to spontaneously decompose. In connection with the process according to the invention, the term sodium percarbonate therefore includes both uncoated sodium percarbonate that has been produced by any process, for example, by a crystallisation process or a fluid-bed spray-granulation process, and sodium percarbonate that has been coated with one or more stabilizing covering layers, the covering layers possibly containing one or more stabilizing components such as, for example, alkali sulfates, sodium carbonate, sodium bicarbonate, waterglass, borates and perborates, magnesium sulfate, and magnesium salts of carboxylic acids.
JP-A-57-42510 discloses a process for storing sodium percarbonate in which cooling air at a temperature not exceeding normal room temperature is passed into the storage container at a relative humidity of at most 70% and, while the sodium percarbonate particles are being cooled, the atmosphere in the storage container is at the same time replaced by the cooling air. The aim of this process is to stabilize the sodium percarbonate, which is transferred at elevated temperature from a dryer into the storage container and is thus contained in an atmosphere having a high relative humidity of air. The contents of the silo are cooled by the passage of cooling air through the product stored in the silo and at the same time the water content is decreased. In this way the storability is increased and the tendency to agglomerate is lessened. However, from this document no suggestions can be inferred as to how sodium percarbonate can be stored for a long period of time in a large silo without posing a safety hazard. Evidently, the problems associated with the spontaneous decomposition of sodium percarbonate in a bulk container, with the liberation of heat, were not known and accordingly no solution to the problem was proposed.
EP Patent 0 396 175 B1 is directed towards a process for preserving sodium perborate monohydrate particles, which are stored in bulk in an impermeable chamber at a temperature of between 10 and 65xc2x0 C. Here, dry air is passed through the stored material throughout the entire storage period. The dew point of the dry air in this case is to be in particular below xe2x88x9220xc2x0 C. and the quantity of dry air is to be between 1 and 8 Normxc2x7m3/(hxc2x7tNaPB). This document contains no suggestion that the process may also be transferred to the storage of sodium percarbonate in a silo. The disadvantages of this process are that, throughout the entire storage period, a large quantity of dry air having a very low dew point has to be passed through the stored material.
As used herein, the term xe2x80x9cNxe2x80x9d is an abbreviation for xe2x80x9cNormxe2x80x9d and means xe2x80x9cat standard conditionsxe2x80x9d of 1013 mbar and 20xc2x0 C. The term xe2x80x9cmxe2x80x9d stands for xe2x80x9cmetersxe2x80x9d, the term xe2x80x9chxe2x80x9d stands for xe2x80x9choursxe2x80x9d, and the term xe2x80x9ctNaPcxe2x80x9d stands for tons of sodium percarbonate, NaPc.
The present invention provides a process for the safe storage of sodium percarbonate, including coated sodium percarbonate, in bulk containers, in particular in large silo works. The process makes it possible to economically maintain the stability of sodium percarbonate as constant as possible during storage, to avoid critical storage conditions caused by spontaneous heating inside the storage container, and to reliably bring these conditions under control should they arise.
The inventive process, which allows for the safe storage of sodium percarbonate in a bulk container that is substantially closed to the ambient atmosphere, is characterized in that the temperature is measured at at least one position inside the sodium percarbonate stored in bulk and with an increase in temperature within the range of 0.5 to 10xc2x0 C. per day, in particular 1 to 5xc2x0 C. per day, dry air is passed periodically or continuously through the bulk material at least until the increase in temperature has fallen to a value of less than 1xc2x0 C. per day, in particular less than 0.5xc2x0 C. per day.